Now to growing fears about a mysterious new strain of bird flu. Three new cases confirmed today, all in china. Tonight health fishes here in the u.S. Are tracking every transmission. Abc's dr. Richard besser is here with us. You were at the cdc, you've hunted down deadly viruses and you've been talking to them about this. They are worried. I talked to my friend who heads up the flu division. I've never heard her more worried about a strain of bird flu. That's why they're reacting so aggressively. In china, 500 hospitals and 400 labs are on the lookout. Poultry markets in shanghai are closed. 20,000 have been destroyed just in case. All because a new bird flu has infected 24 people, and seven of them are dead. Bird flus are not unusual, but they can only move from one bird to another. This viruso have mutated. Now able to leave birds and attach itself to human cells. The darkest fear? The virus mutates again and again, and begins to jump from person to person to person. And then an infected person could get on a plane in shanghai. It's not hard to see what happens next. A pandemic, spreading across the world at the speed of a cough or sneeze because we have no I'm munt to the new flu. It seems so far away, but as you point out, it can move so fast. Yeah, I've been talking to them every day. It's been a long time since I've seen people at cdc so concerned about a new infection. I know you'll stay on top of this.

This transcript has been automatically generated and may not be 100% accurate.

{"id":18909723,"title":"Cases of New Bird Flu Strain Reported in China","duration":"1:36","description":"Health officials track transmissions of the new strain of the virus that has killed six.","url":"/WNT/video/cases-bird-flu-strain-reported-china-18909723","section":"WNT","mediaType":"default"}